1. Statement of the Technical Field
The inventive arrangements relate generally to the field of satellite systems, and more particularly to a constellation of satellites.
2. Description of the Related Art
Earth satellites are used for a wide variety of civilian and military purposes. In the civilian realm, these purposes can include weather, communications, mapping, and resource exploration. Satellites used for military purposes can serve as communication links, surveillance platforms and weapons systems. Global positioning system (GPS) satellites are commonly used for both civilian and military purposes.
In many instances, two or more satellites must operate in a cooperative arrangement in order to satisfy mission requirements. Such groups of satellites are called constellations. For example, in the field of communications, multiple satellites may be required to provide radio signal relay coverage over a particular political or geographic region. Similarly, GPS systems typically require terrestrial GPS receivers to receive signals simultaneously from multiple satellites. U.S. Pat. No. 5,551,624 discloses a constellation of telecommunications satellites that exclusively makes use of prograde orbits to provide 24-hour cellular telephone communication coverage over a predetermined latitude range around the world. This predetermined latitude range can be thought of as a band that encircles a selected portion of the earth. This band defines a zone of operational utility for a constellation of satellites and the purpose of each mission is to act or do something within this band. For example, a band could extend from the equator to the north pole. In that case, the band would include the entire northern hemisphere of the earth.
A satellite's motion is confined to a plane that is fixed in space. This plane is often referred to as the orbital plane. The orbital plane always goes through the center of the earth, but may be tilted any angle relative to the equator. Inclination is the angle between the orbital plane and the equatorial plane. Alternatively, a vector h can be defined perpendicular to the orbital plane. Similarly, a vector K can be defined aligned with the celestial north pole of the earth. The angle defined between the vector h and the vector K is also the inclination angle of the orbital plane. The operational band of a satellite as described above is substantially determined by the inclination angle of the satellite's orbit and orbit altitude. For example in the case where a low earth orbit (LEO) satellite (altitude between 1000 km to 3000 km) has an orbit with a 60° inclination, the band would run from about 55° to 65°. Similarly a satellite that has an orbit with a 30° inclination would have an operational band that extends from about 25° to 35°. Those skilled in the art will appreciate that the operational band is not exactly symmetric about the inclination angle, but for a circular orbit, the foregoing estimates are reasonably accurate. Notably, prograde orbits have inclination angles between 0° and up to 90°. Retrograde orbits have inclination angles between 90° and 180°.
Relative motion for a prograde orbit will be opposite to the relative motion for a retrograde orbit. Conventionally, satellite motion for a prograde orbit is east to west. When the inclination angle is rotated such that a prograde orbit is produced, the same orbital rotation will cause the satellite to have a west to east relative motion with respect to the earth.
In the field of military surveillance, more than one satellite may be required for ensuring that a satellite will have a view of a particular geographic location on a regular and timely basis. For example, missile detection and tracking systems can require nearly constant monitoring of large geographic areas located at various locations around the earth. In those instances where global or global sector (a band formed by upper and lower latitude lines on the earth) coverage is required, a number of satellites are required so that at least one satellite can be seen from every point within the global or geodetic sector of interest. In addition to ground coverage, an ephemeris dataset can be defined that describes a trajectory of interest.
Other considerations of importance when selecting the satellite constellation orbits include the need to 1) maximize satellite-to-ground terminal connectivity (minimizes latency for all ground functions), 2) maximize satellite-to-trajectory visibility time (enhances trajectory discrimination), 3) minimize the range between satellite-to-trajectory (for optimal sensor resolution), and 4) maximize the design life of each satellite by avoiding intense Van Allen Belt ionization regions and cosmic radiation surrounding the earth.
It is well known that satellites and the launch vehicles required for such satellites are very expensive. Accordingly, it is essential to select a constellation that accomplishes the goals of the mission with a minimum number of satellites. The constellation chosen should also have orbits that support the longest design life in terms of orbit decay, and radiation damage. Another factor to consider when selecting satellite constellations is the need in certain instances for satellite to communicate with ground controllers, satellites, and other resources. This can be accomplished directly via inter-satellite links (ISLs) that relay the ground communication to the desired satellite within the constellation. Ideal conditions for ISLs can be established and maintained because relative satellite motion within an orbit plane is nearly zero.